In recent years, emission control is being tightened in view of conservation of environment. Specifically, an amount of fuel, which evaporates in a fuel system such as a ventilation apparatus and leaks to the outside, is regulated, as well as an amount of exhaust gas emitted from a vehicular internal combustion engine or the like. According to the regulation defined by the Environment Protection Agency (EPA) and the California Air Resource Board (CARB) in United States, it is required to detect vapor of fuel leaking through a small opening (leakage hole) in a fuel tank.
According to U.S. Pat. No. 5,890,474 (JP-A-10-90107) and US20040000187A1 (JP-A-2004-28060), a conventional leakage detecting device for an evaporating fuel processing apparatus has a ventilation apparatus that includes a fuel tank, a canister serving as an absorbing filter, and a purge control valve.
The inside of the ventilation apparatus is pressurized or de-pressurized using a pump to generate pressure difference with respect to the outside thereof. In this situation, pressure varies in the ventilation apparatus, and this pressure variation is compared with a reference pressure variation, which corresponds to a reference leakage hole, so that leakage arising in the ventilation apparatus is determined.
According to U.S. Pat. No. 5,890,474, an electric pump pressurizes to produce the reference pressure variation. The reference pressure variation and the pressure variation in the ventilation apparatus are detected in accordance with load fluctuation in a motor that drives the electric pump. Voltage applied to the motor or rotation speed of the motor is detected as the load fluctuation in the motor.
According to US20040000187A1, a brushless motor is used in an electric pump to enhance the lifetime of the electric pump. This structure includes a first intake circuit, which intermediately has the reference leakage hole, and a second intake circuit, which communicates with the ventilation apparatus. Positive pressure or negative pressure generated using the electric pump is switched between the first intake circuit and the second intake circuit using the switching valve. The reference pressure variation and the pressure variation in the ventilation apparatus are alternatively detected using the switching valve, so that a period needed for detecting leakage in the ventilation apparatus can be reduced.
However, in the above conventional structures, the fuel tank is pressurized or de-pressurized when leakage in the ventilation apparatus is detected. Accordingly, a pressure range in the pressurizing or the de-pressurizing using the electric pump is limited for protecting the fuel tank and for accurately detecting leakage in accordance with the pressure variation. The discharge performance of the electric pump varies due to aging, and varies corresponding to temperature characteristic of the motor. Accordingly, the pressure variation may not be limited within the pressure range due to the variation in the discharge performance. Leakage detection may be quickly switched from detecting the reference pressure variation to detecting the pressure variation in the ventilation apparatus alternatively in this order using the switching valve, for example. However, even in this case, the discharge performance of the electric pump may vary corresponding to the temperature characteristic of the motor while detecting the reference pressure variation and detecting the pressure variation in the ventilation apparatus.
Besides, when voltage of a vehicular battery varies, discharge performance of the electric pump may vary. Accordingly, the electric pump may be operated by controlling power supply at a constant voltage. However, even in this case, the discharge performance needs to be initially adjusted within a small pressure range in an assembling process such that the pressure variation is limited within the pressure range in an actual operation.